Does this patch remove the last uses of those functions? If yes, then I would recommend removing these functions, otherwise it is just too easy to find them with grep or autocompletion while missing the deprecation comment.

Maybe go with the aarch64 solution of exposing a getProcFamily() function (that forces you to look around for the enum and write a little bit more which may be enough to get people reading the comment and thinking :)

Hm, I think you're right about this, it probably shouldn't be a subtarget feature. I'll update the patch to undo this part for now, and maybe in time the isSwift check will evolve into an isDarwin or something.

These aren't the last remaining uses of these functions. There are also some in review in D21797 and D21798, and even more that I've documented here: https://llvm.org/bugs/show_bug.cgi?id=27992
I've marked the functions as obsolete for now because I intend to put this on hold for a while, and I don't want to come back and find 20 new uses besides the ones in PR27992 :)